The need for increased communications bandwidth has led to progressive increase in communications speeds, with single wire serial channel rates now measured in tens of gigabits per second. Ideally, a multiwire communications channel could deliver even more bandwidth by sending entire “words” of data in parallel across multiple channel elements, but such schemes are inevitably constrained by the differential propagation delays of the various channel elements. As the variations in arrival time for the various data elements becomes a significant percentage of the transmission unit interval for the channel, the time window during which an entire valid data word may be captured shrinks, and eventually closes.
In an ideal world, a multiwire communications receiver would incorporate detailed amplitude and timing detection apparatus on each individual wire input, allowing every variation in signal strength or timing to be measured, analyzed, and mitigated. Unfortunately, real-world systems operate under constraints on power, complexity, and speed, thwarting introduction of any but the most essential detection components. In practice, a multiwire receiver may be limited to a sampler capturing receive data from each wire, wire pair, or wire group comprising a data channel, and some minimal means to maintain receive clock synchronization. Thus, the effects of differential propagation time or “skew” among the input signals will be experienced as reduced signal quality, in particular as horizontal reduction of the eye opening in a time-versus-amplitude received signal “eye” diagram, with no additional information as to how the problem might be mitigated.
As one example, consider a two wire differential circuit terminating in a single differential line receiver. If one of the two wires has a significantly different propagation time than the other, the time interval within which the differential line receiver output is valid will be reduced, but there is no way of knowing which of the two input signal paths is the problem. Various solutions have been proposed in the art, generally incorporating adjustable delay elements in the received wire signal paths, combined with trial-and-error delay adjustments seeking to “tune” those signal paths for maximum signal quality.
The situation is somewhat better for receivers that derive receive clock information from received signal transitions. As transitions may occur on any received signal channel, each channel will typically incorporate some minimal clock-data-alignment or CDR apparatus, typically comprised of an additional sampler configured to provide “early/late” feedback for the local sampling clock source, relative to input signal transitions. However, as shown by the differential receiver example above, one timing datum per receive channel may not be sufficient to unambiguously resolve the source of timing errors to the individual wire path level.